ToggleButton myToggle = new ToggleButton();

Timer myTimer = new Timer() {
  @Override
  public void run() {
    if (myToggle.isDown()) {
      // Perform repeating action.
    } else {
      cancel();
    }
  }
};

myToggle.addValueChangeHandler(new ValueChangeHandler<Boolean>() {
  @Override
  public void onValueChange(ValueChangeEvent<Boolean> event) {
    if (event.getValue().booleanValue() == true) {
      // Perform the repeating action twice/second.
      myTimer.scheduleRepeating(500);
    }
  }
});